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  <script>
    var largeGroupPositions = function (s) {
      const ret = [];
      const n = s.length;
      let num = 1;

      for (let i = 0; i < n; i++) {
        if (i === n - 1 || s[i] !== s[i + 1]) {
          if (num >= 3) {
            ret.push([i - num + 1, i]);
          }
          num = 1;
        } else {
          num++;
        }
      }
      return ret;
    };
    console.log(largeGroupPositions("abbxxxxzzy"));
  </script>
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